In U.S. Pat. No. 3,953,134, which is assigned to the assignee of the present invention, there is described an optical wheel alignment instrument which may be used for measuring the toe angles of the wheels of a vehicle. When using that patented instrument to align the front wheels of a vehicle, the steering wheel is preferably locked in the straight ahead position, and the toe angles of the two front wheels are then measured and set relative to one another and to the central longitudinal axis of the vehicle. If one of the front wheels is either forward or rearward of the other, a condition known in the art as set-back, when the vehicle is later driven straight ahead, the steering wheel will be at an angle. This same result may also occur if the thrust axis of the vehicle is not parallel to the central longitudinal axis of the vehicle.
When using prior art methods of wheel alignment, the vehicle should be road tested after being aligned, and if the vehicle does not track in a true straight ahead direction when the steering wheel is centered, the alignment technician should then correct the condition by making equal and opposite adjustments of the toe angles of the two wheels being aligned. The extent of such adjustments has been a matter of judgment based on the experience of the technician and is a trial and error process. Instruments have, however, been on the market for measuring the toe angles relative to the tracking axis of the vehicle, but the high cost of such instruments, the difficulty in maintaining them calibrated, and the time required to make such measurements have limited their use. Consequently, the trial and error readjustment of the toe angles following a road test of the vehicle remains the most common method of adjusting toe to correct for wheel set-back.